The method which is the object of the invention relates to the detection, location and assessment of faults existing on the walls of metal tubes having internal helical ribs.
In particular, this method relates to faults present in tubes used mainly for the production of heat exchangers for fuelled power stations. According to their conditions of use, such tubes may be made from non-alloyed, stainless or refractory steels or from other metals or alloys.
In known manner, systematic ultrasonic inspections are carried out on these tubes to make sure that any longitudinal or transverse cracks formed on the outer or inner walls do not attain a depth which is likely to lead to their subsequent breakdown.
At the present time, the existence of such cracks on such tubes are detected by means of ultrasonic waves. According to the state of the art, FIG. 1 shows how faults are looked for on the outer wall of a metal tube 1 which has no ribs. An ultrasonic emitter-receiver transducer emits an ultrasonic beam having an inclined axis at an angle "i" in relation to the radius of the tube of axis Xl in the sectional plane of the drawing. The tube is propelled in relation to the transducer ER and performs a relative movement of rotation about its axis while sliding along this axis. It can be seen that an external longitudinal crack R is detected by an ultrasonic beam emanating from the transducer ER which travels the path ABC in the wall of the tube with total reflection at B, a part of the beam being returned to the transducer by the fault R along the path CBA.
Thanks to the known electronic means, the amplitude of the signal corresponding to the fault R may be determined and compared With the amplitude of a standard fault which is stored in the memory. It is then possible to eliminate those tubes in which the cracks, detected by ultrasound, provide signals of an amplitude equal to or greater than a threshold corresponding to the amplitude of the signal given by the standard fault.
Tests carried out in order to apply the same method of ultrasonic detection and inspection for external or internal cracks and fissures on internally ribbed tubes have shown that the existence of ribs resulted in quite substantial fluctuations in the amplitude of the fault signals received by the transducer. The outcome is that it was not possible reliably to compare this amplitude with a specific threshold. It was then necessary to adopt quite a wide safety margin, entailing the risk of rejecting tubes of good quality or, on the other hand, to reduce the safety margin and take the risk of accepting tubes which do not comply with the standards or specifications.
The possibility was sought of perfecting a method of using ultrasonic waves to detect and check for faults in metal tubes having internal ribs and which make it possible to take into account fluctuations in amplitude observed during the course of detection of one and the same fault so that this fault may be compared with a standard by automatically employing electronic signal processing means.